Unit 1 Test

Question 19 • AP Biology - SC5182 A

A runner picks up pace on a warm day and starts sweating profusely. What property of water explains how sweating helps the runner continue running?

Answer

Since water molecules are polar, they adhere to the surface of the skin, which allows cooling. The skin’s outer layer of dead cells enables the adhesion.

At elevated body temperature, the hydrogen bonds in water break, which allows water to escape as vapor. This causes heat loss in the body, which lowers body temperature.

Hydrogen bonds rearrange at higher temperatures and form a lattice structure that requires energy. This energy use lowers the heat in the body.

Sweating is due to the high heat capacity of water. When sweating increases, heat is released and the body temperature is lowered.

Explanation

Sweating cools the body because water has a high heat of vaporization: energy is needed for sweat to evaporate. That energy comes from heat at the skin, lowering body temperature. Choice B is the closest: evaporation involves overcoming attractions between water molecules, including hydrogen bonds, so water escapes as vapor and carries heat away. High heat capacity in choice D describes how much energy water can absorb before its temperature changes, not the cooling effect of evaporation.

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